Various techniques are utilized to provide carrier and symbol synchronization signals for burst communications with MSK modulation. Of particular interest is the DeBuda synchronization circuit for fast frequency shift keying (FFSK). See Rudi DeBuda, "Coherent Demodulation of Frequency-Shift Keying with Low Deviation Ratio", IEEE Transactions on-Communications, June 1972, pages 429-435. The only difference between the FFSK technique described by DeBuda and the MSK technique applicable herein is that the FFSK process has some pre-coding operation prior to modulation, such as differential encoding. For the purposes herein, this procedural difference can be ignored.
The MSK modulation imparts a continuous phase advance or delay upon the carrier signal depending upon the value of the transmitted bit sequences. This change in the carrier signal phase imparts a secondary modulation upon the signal which must be removed. Specifically, this modulation is manifested in the polarity of the MSK signal at either of the higher or lower frequency tones; these tones can have a positive or negative polarity.
In order to remove this modulation, synchronizer circuits such as the one taught in the DeBuda article, above, employ a squaring technique to remove the effect of the modulation on the polarities of the two MSK tones. This squaring technique consequently results in the doubling of the frequencies of the two MSK tones.
Although the signal squaring technique removes this secondary modulation effect, it also introduces an approximately 6-dB loss in signal to noise ratio (SNR), and further results in a four-state phase ambiguity. The 6-dB loss in SNR results from requisite filtering at the second harmonic of the two MSK tones rather than at the fundamental frequencies. Each of the two filtering processes results in another 6 dB loss since only one frequency is present at any time while noise is always present in approximately equal amounts at both upper and lower frequencies. Since the signal is filtered through a pair of parallel branches (higher and lower frequencies) the SNR for each filter is reduced by a factor of 2, or 6 dB. Thus the necessity of filtering the MSK signal at the harmonic frequencies introduces a pair of 6 dB losses in SNR, for a total SNR loss of 12 dB.
Furthermore, the 4-state ambiguity in phase can only be removed by differential coding or additional preamble overhead in the form of a known bit sequence.